Calcium phosphates are known in the art as physiologically acceptable biomaterials potentially useful as hard tissue prosthetics. The most widely studied of these are hydroxyapatite and tricalcium phosphate. When these materials are shaped and made porous they can be used alone or as a supplement or extender with bone for hard tissue prosthetics. Under appropriate conditions and with an appropriate form of calcium phosphate, the calcium phosphate is resorbed and new bone growth results. Calcium phosphate biomaterials can be molded by compaction under high pressure. Pore formation of molded calcium phosphate biomaterials is generally achieved by compaction of calcium phosphate powders containing naphthalene followed by removal of the naphthalene by leaching or sublimation. Hydrothermal exchange of marine coral structures (i.e., calcium carbonate for calcium phosphate), and decomposition of hydrogen peroxide have also been employed to generate pore filled structures.
The dense or "green" forms of the calcium phosphate implant materials have mechanical properties equal to or exceeding that of natural bone, but their respective porous forms do not, thus severely limiting their usefulness as hard tissue prosthetics.
The art teaches that natural and synthetic polymers can be used in conjunction with various inorganic mineral fillers such as porous and powdered forms of calcium phosphate to enhance their mechanical properties for use as hard tissue prosthetics or to enhance the bonding of metallic or plastic prosthetics to natural tissue. Natural polymers include collagen (U.S. Pat. No. 4,192,021) and gelatin (German Pat. No. 2,812,696). Synthetic polymers include polyacrylates, poly(methylmethacrylate), polyethylene, polysulfones, polyamides, polyesters, polytetrafluoroethylene, and polycarbonates (Great Britain patent application Ser. No. 2,031,450A); polyacetates and polyglycolates (U.S. Pat. No. 4,192,021); epoxides, polyacrylamide, polypropylene, polyurethanes, polyacetals, silicone resins, and furan resins (U.S. Pat. No. 4,222,128); polyvinyl pyrrolidone, polyvinyl alcohol (U.S. Pat. No. 4,263,185); and a cross-linked pentapeptide (U.S. Pat. No. 4,187,852). The natural polymers and some of the synthetic polymers are resorbable, i.e., biodegradable.
The art also teaches that nontoxic water soluble substances such as sodium chloride can be incorporated into a mixture of powdered acrylic polymer, liquid monomer, and other ingredients in a mold and the mixture polymerized to produce a shaped composite. The composite can then be made porous by leaching the sodium chloride with water (U.S. Pat. No. 4,199,864).
The various polymer-calcium phosphate composites are prepared in a number of ways including blending calcium phosphates with polymeric binder and subsequent molding (Great Britain Pat. No. 1,593,288); impregnation of sintered, porous calcium phosphate with polymers under vacuum (Great Britain Pat. No. 1,593,288); impregnation of a porous calcium phosphate body with the melt or solution of prepolymers and solidifying the polymers by further polymerization or curing in the pores or by evaporation of the solvent (U.S. Pat. No. 4,192,021); impregnation of a porous calcium phosphate body with a very reactive monomer like an .alpha.-cyanoacrylate or monomer and catalyst and polymerizing by heating (U.S. Pat. No. 4,192,021); compression molding of an intimately blended, finely powdered mixture of polymer and calcium phosphate (U.S. Pat. No. 4,192,021); and embedding ceramic calcium phosphate particles into resins where the calcium phosphate particles have previously been coated with a resin-affinic material to ensure good bonding to the resin, or copolymerizing precoated particles with the resin monomers (German Pat. No. 2,620,907).
Calcium phosphate-polymer composite materials can also be used in conjunction with metallic or plastic prosthetics to facilitate adhesion and bone growth around the prosthetic (German Pat. No. 2,905,647). The composite can also be applied as a coating, for example, to an anodized titanium/aluminum/vanadium alloy hip prosthetic (Great Britain Pat. No. 1,593,288). The essential element in anchoring prosthetic devices appears to be the induction of new bone growth around the device by assuring that contact with the surrounding tissue is through a sheath of, or a surface laden with, bioactive calcium phosphate.